By the manufacture of glass products, particularly glass packages in the shape of bottles and jars, it is important that the blanks that are fed to the forming machine are mutually similar as far as possible with regard to both weight, temperature and other properties. The glass mass is molten in a furnace and is withdrawn therefrom through a plurality of channels, so called forehearths. Here, the desired properties of the withdrawn glass should be secured, beyond all by an exact control and adjustment of the temperature of the glass mass.
To achieve the exact control of the temperature of the glass mass there must exist a possibility to both heat and chill the glass mass. This may be achieved in different ways. The heating may be achieved either by installation of burners in the walls of the forehearth for combustion of e.g. gas, or by electrical heating. In the last mentioned case the heating may be achieved by means of electrodes, mounted in the side walls of the forehearth to protrude into the glass mass and adapted to heat the glass by direct current passage through the glass mass between the electrodes, or by means of resistance elements which are suitably disposed above the glass mass. If the heating is to take place by combustion it is also necessary to provide the forehearth with means for admission of combustion air and exhaust of combustion gases.
A certain amount of cooling of the glass mass always takes place by conduction through the forehearth walls and through radiation to the roof and conduction therefrom. To achieve an additional cooling, which is often desirable, two principally different methods have been utilized, via, direct and indirect air cooling, respectively. On direct air cooling the cooling air is blown into the forehearth directly above the surface of the glass mass. This results in a comparatively good cooling, but the cooling effect is achieved primarily on the glass surface, which brings about a risk for skin forming on the surface.
Indirect cooling may be achieved by building in a plurality of cooling channels in the roof of the forehearth. In this way, however, rather low cooling effects are achieved due to the low heat conductivity of the high temperature resistant material from which the forehearth is constructed.
To improve the cooling effect forehearths with openings in the roof construction have been designed. These openings lead to channels for cooling air, and where the openings open into the cooling air channels the openings are covered by a lid of heat resistant material. A certain increase of the cooling effect has been achieved in this manner at the expense of a comparatively complicated design. Due to the viscosity of the glass mass and the cooling effect obtained through the bottom and the walls of the forehearth there results an uneven flow and temperature distribution over the cross-section of the forehearth. Very great variations may occur if this is not counteracted by either cooling in the middle of the forehearth or heating along along the side edges, or a combination thereof. To obtain this the above mentioned cooling channels or -openings may be placed essentially above the middle of the forehearth. The roof of the forehearth may be designed in such a manner that a plurality of channels are formed, and cooling air may be blown through these channels as desired. To achieve a heating, particularly of the side edges of the forehearth, designs may be made where the effect of combustion of e.g. gas is directed as far as possible towards the outer edges of the forehearth.
A further factor that has an influence on the temperature variations and the need for cooling and/or heating is the quantity of glass per time unit that is withdrawn through the forehearth. Here, great variations may occur in one and the same forehearth, depending on which particular product in under manufacture, and the production rate. The difference between the greatest and the least glass withdrawal rate may be a factor of five. This puts great demands on the possibility to change the cooling and the heating, respectively, within a broad interval. The proposed measures have been enough to diminish the temperature variations to a certain extent but, despite the described measures, the difference between the highest and the lowest temperature in a cross-section over the outlet of the forehearth may amount to 40.degree. C. However, also this variation may be considered to be great, and gives rise to problems.